Method for manufacturing an expandable stent

ABSTRACT

A method of making a balloon expandable braided stent with a restraint to initially prevent self-expansion and the resulting product. Multiple strands of a resilient metal or plastic are braided to form a tubular configuration of a predetermined outside diameter which assumes a lesser diameter when the tubular stent is longitudinally stretched. When in its stretched condition, it is coated with a polymeric material which is then cross-linked to effectively “freeze” the intersections of the braided structure holding it in its reduced diameter configuration. The tubular stent is designed to be placed within a body vessel using a balloon stent delivery catheter. When the balloon surrounded by the braided wire stent is inflated, the stent expands initially to an extent to break the bonds of plastic material between the intersections of the strands, thereby permitting self-expansion to take place. The coating may also comprise a hydrogel or an elastomeric impregnated with the water soluble particles which softens and/or deteriorates upon exposure to an aqueous media, such as blood.

This is a Divisional of application Ser. No. 08/905,704, filed on Aug.4, 1997, now U.S. Pat. No. 5,899,935.

BACKGROUND OF THE INVENTION

I. Field of the Invention

This invention relates generally to the manufacture of vascular stents,and more particularly to a method of making an otherwise self-expandingstent balloon expandable.

II. Discussion of the Prior Art

Various types of stents are described in the prior art and theygenerally fall into one of two classes, namely, self-expanding stentsand balloon expandable stents. A common type of self-expanding stent isreferred to as a “Wallstent® Endoprosthesis” and is described in U.S.Pat. No. 4,655,771 to Wallsten, which is incorporated herein byreference in its entirety. It comprises a braided tubular structure ofmetal wires or monofilament plastic strands. The tubular structure ischaracterized by a longitudinal shortening upon radial expansion.

In use, such a stent may be longitudinally extended to achieve a reducedradial diameter and placed within the lumen of a delivery catheter. Thedelivery catheter may then be advanced through the vascular system untilits distal end is located proximate a stenotic lesion to be stented. Thestent is then deployed out the distal end of the catheter and whenunconstrained by the catheter, it self-expands into contact with thevessel with sufficient radial force so that it is maintained in theblood vessel for an extended period of time. A suitable delivery devicefor a Wallstent® Endoprosthesis is shown in U.S. Pat. No. 5,026,377 toBurton et al.

An example of a balloon expandable stent is the “Palmaz™” balloonexpandable stent described in U.S. Pat. No. 4,733,665. A balloonexpandable stent may comprise a fenestrated tube of material having alow modulus of elasticity and with substantially no memory property. Thefenestrations through the wall of the tube are such that when the tubeis placed over the deflated balloon on a balloon delivery catheter andthen is routed to the location in the vascular system where it is to beused, the inflation of the balloon deforms the stent from a reduceddiameter to a larger operating diameter. The balloon is then deflatedand withdrawn, leaving the stent in place.

A need exists for a stent that has the self-expanding characteristics ofa braided stent, but which can be delivered over a balloon for accuratepositioning and deployment. The present invention fulfills that need.

SUMMARY OF THE INVENTION

The method of the present invention provides a balloon expandable stentthat comprises a tube of braided wire having a relatively small outsidediameter when in a longitudinally extended state and which normallyself-expands to a larger outside diameter when in a longitudinallycontracted state. As used herein, the “free state” of a self-expandingstent is the state that is reached when no external forces are appliedthereto. This free states corresponds to a radially fully expandedstate. The braided tube is coated with either a brittle material or amaterial that will readily soften when exposed to body fluids forinitially restraining the braided wire tube from self-expanding to thelarger outside diameter when external forces are removed. When thecoated braided wire tube is concentrically disposed over the balloon ofa stent delivery catheter and the balloon is inflated, the restrainingforce imposed by the brittle coating is effectively broken, allowing thedevice to self-expand.

It has also been found expedient to incorporate into the coating of thepolymeric material an effective amount of water-soluble solid particlesthat, when dry act to reinforce the coating, and when placed in anaqueous medium, such as blood, will elute with time and thereby lessenthe restraint imparted by the coating on the intersections of the wirescomprising the braided wire tube, permitting expansion thereof. Theparticles may comprise a drug.

The method of manufacturing the balloon expandable stent comprises thesteps of first braiding a plurality of resilient metal or plasticstrands having a memory property into a tubular structure of apredetermined outer diameter. Then the resulting tubular structure isheat treated on a mandrel at a suitable temperature to cause stressrelief (generally about 500-600° C. (For Elgiloy®) thereby causing thetubular structure to conform to the shape of the mandrel. Next, thetubular structure is stretched longitudinally to reduce its outerdiameter and, while it is clamped and held in the stretched condition,the tubular structure is coated or otherwise treated with a brittleplastic or other material capable of freezing the intersections againstmovement. If a cross-linkable material containing a water soluble powderis to be used, the material is cross-linked either by application ofheat and/or radiation, such that the coating holds the stent in itsstretched state and precludes self-expansion thereof to itspredetermined outer diameter, even when the force used to longitudinallystretch the tubular structure is removed. When used, the tubularstructure may be concentrically positioned over an uninflated balloon ona balloon delivery catheter and, following the passage of the balloonand stent through the vascular system to the site of a lesion, theballoon is inflated to radially expand the tubular structure sufficientto rupture or disintegrate the coating and allow self-expansion of thetubular structure. The destruction of the integrity of the coating isachieved by the use of a hydrogel as the coating or the inclusion ofsoluble particles in the polymeric matrix and the subsequent exposure ofthe coated device to an aqueous medium.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is an enlarged side elevational view of a self-expanding stentthat has been treated with a material in accordance with the method ofthe present invention to lock the intersections of longitudinallystretched stent;

FIG. 2 illustrates the stent of FIG. 1 after it has been balloonexpanded;

FIG. 3 is a drawing showing a coated stent surrounding an uninflatedballoon on a delivery catheter and contained within a protective,moisture impervious sheath;

FIG. 4 is a flow chart illustrating the steps involved in one method ofmaking a balloon expandable braided stent incorporating a polymericrestraint; and

FIG. 5 is a flow chart of the steps involved in using the balloonexpandable self-expanding stent of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIGS. 1 and 2, the balloon expandable braided stentconstructed in accordance with the present invention is indicatedgenerally by numeral 10 and is seen to comprise a plurality of rigid,but flexible strands 12, 13, 14 and 15, each of which extends in a helixconfiguration with the center line of the resulting tubular body as acommon axis 16. A first group of strands are wound in the samedirection, but are axially displaced relative to one another. Thesestrands cross with another set of strands that are also axiallydisplaced with respect to one another, but which have the oppositewinding direction.

The braiding machine used to interweave the strands may be controlled soas to include a sufficient number of strands in the braid as well as apick to provide a stable configuration corresponding with the outsidediameter (O.D.) of the stent body when it is unconstrained (free state),as well as the angle of intersection of the crossing strands. Thestrands themselves may be metal, such as stainless steel, anickel-titanium alloy or clad composite construction as shown in U.S.Pat. No. 5,628,787 to Mayer or, alternatively, may be a suitable medicalgrade plastic monofilament.

The braided structure is such that the application of a longitudinalstretching force, such as represented by the force vectors 18 and 20 inFIG. 1, results in an elongation of the stent body and a correspondingreduction in its radial dimension. The device is self-expanding in thatwhen the longitudinal stretching forces are relieved, the tubular bodywill diminish in length while expanding radially.

A conventional self-expanding braided stent of the type heretoforedescribed may be implanted within a tubular body vessel, such as anartery, by first radially compressing it so as to reduce its diameter tothe point where the stent can be inserted into the lumen of a stentdelivery catheter. The delivery catheter containing the stent may thenbe routed through the vascular system until the distal end thereof isdisposed proximate the site of a lesion or the like to be bridged. Anouter sleeve is then withdrawn to expel the compressed stent from thedistal end of the delivery catheter and when no longer constrained bythe delivery catheter, the stent will self-expand, radially, to presenta significantly larger outside diameter selected so that it willcontinue to press itself against the interior wall of the blood vesselin question.

The flow chart of FIG. 4 shows an example of the steps involved inmaking the stent of the present invention. To render the braided stentof the present invention balloon expandable and, therefore, easier toposition, the woven stent, after it comes off the braiding machine andis heat treated, is then clamped at opposite ends and longitudinallystretched as illustrated in FIG. 1. While in this stretched condition,the stent is dipped in or sprayed with a solution of a suitable polymerin a volatile solvent, spot glued with such a solution or a meltpolymer, and then air dried or heated to drive off the solvent and tocure the polymer to effectively “freeze” the stent in its elongated,reduced diameter configuration even when the stretching forcesrepresented by the vectors 18 and 20 are removed. The cured polymericcoating 24 (FIG. 3) creates a restraint between the opposed sets ofhelical windings at their points of intersection and preventsself-expansion from taking place. Suitable polymers for this methodinclude hydrogel materials.

An alternative method for making the device includes, while the stent isin the stretched condition, applying a brittle polymer or inorganicmaterial to the stent by vapor deposition or ion beam assisteddeposition. Suitable materials for vapor deposition include parylene andpyrolytic carbon which are quite brittle. A suitable material for ionbeam assisted deposition includes ceramic coatings which are also quitebrittle.

It is further contemplated that a water-soluble powder be blended withan elastomer, such as silicone, which serves as a reinforcer when thecoating is dry. When the coated stent is exposed to blood, the powderparticles will dissolve, weakening the coating's restraining effect onthe elongated, radially reduced stent, thus facilitating its ability tolater self-expand. Further, the powder may comprise an anti-plateletagent, an anti-coagulant agent, an anti-microbial agent, ananti-proliferative agent, an anti-metabolic agent and ananti-inflammatory agent, a drug for enhancing endothelial growth forholding the stent in place or for inhibiting thrombosis. When dry, thedrug or water-soluble powder particles reinforce the coating. Uponcontact with an aqueous media, the drug or water soluble component (e.g.salt, sugar, drug, etc.) will elute from the matrix and gradually softenthe coating and allow later self-expansion of the stent.

A cross-linkable system can also be applied as a polymeric restraint. Inthis case, the coating can be applied while the stent is in its freestate. Thereafter, the coated stent is longitudinally stretched to areduced diameter and then cross-linked to restrain the opposed sets ofhelical windings at their points of intersection.

FIG. 5 is a flow chart showing an example of the steps involved in usingthe improved stent. Rather than deploying the stent of the presentinvention solely by allowing self-expansion from the lumen of a deliverycatheter, the device of the present invention may be placed over anuninflated balloon on a balloon-type stent delivery catheter. With thestent so mounted, the catheter may be routed through the vascular systemuntil the stent is juxtaposed to the site where reinforcement isdesired. Now, as inflation of the balloon takes place and the stentbegins to expand, the brittle coating bonding the stent strandintersections are broken and the stent is allowed to self-expand to thepoint where it presses against the vessel wall with a force sufficientto maintain the stent in place following deflation of the deliveryballoon and removal of the catheter.

To prevent premature self-expansion of the stent when coated with apowder impregnated polymer or hydrogel as it is being advanced throughthe vascular system, it may be contained within a moisture impermeablesheath 22 as shown in FIG. 6. This protective sheath prevents the stentfrom coming into contact with water (blood) before the deployment.Immediately after the withdrawal of the sheath, the balloon is inflated,breaking some of the restrained points or realigning the polymer chainto facilitate the water coming in and the drug or other soluble powdercomponent eluting out, allowing the stent to assume the profile whichthe balloon creates. Since a typical balloon inflation time is 30seconds or more, the coating is further softened during the inflationoperation, and the stent will eventually fully open and recover itsinherent property. Experiments have shown that even without balloonexpansion, the treated stent will recover by itself as the polymermatrix loses structural integrity due to the dissolving powders.

Depending on the amount of the drug or other powder embedded, thesolubility of the drug or other powders and the cross-linked density ofthe coating, the elution rate, and hence the softening speed, can beadjusted. My experiments have shown that the coating can be applied onthe stent, either at a constrained stage or a free stage, stretchedwhile curing and the coated stent will maintain its tubular constrainedshape, after the coating is fully cured.

EXAMPLE

The typical procedure for the stretching cure of soluble powderimpregnated silicone is as follows: A self-expandable Wallstent®Endoprosthesis having a nominal (free state) diameter of 4 mm is coatedwith a 37.5 percent heparin in silicone suspension and then stretchedlongitudinally in a clamping fixture and heat-cured in a convection ovenat a temperature of about 90° C. for a time period of 16 hours. Thestent is then inserted into a small diameter nylon sheath and furthercured during a gamma sterilization process at a dosage of 2.5-3.5 mrad.When extracting the coated stent from the nylon sheath, the stent wasfound to remain constrained. In its constrained state, it was thenmounted on a 3.5 mm balloon catheter. When emersed in water andinflated, the stent was found to recover 90% or more of its nominaldiameter within about 30 seconds and eventually fully opened.

While heparin impregnated silicone coating is preferable, othercross-linkable polymers that may be employed in practicing the inventioninclude cross-linkable polyurethane, cellulose, fluoropolymers,polyolefins, diene polymers and unsaturated polyesters.

The longitudinally stretched, self-expanding wall stent may also betreated with a hydrogel material to temporarily maintain the stent inits stretched state prior to implantation. When the hydrogel is dry, itbehaves like the previously mentioned hard or brittle plastic. Whenexposed to an aqueous media, it will promptly penetrate into thehydrogel to swell the material and soften the coating. The mechanicalproperty of the self-expanding stent will then again dominate. Thus, aprotective sheath is again required during the implantation of thehydrogel coated stent. Typical examples of hydrogel materials useful inthe practice of the present invention are polyvinyl alcohol, polyvinylacetate, polyvinylpyrrolidone, polyacrylamide, polyacrylic acid,polyhydroxyethyl methacrylate, polyethylene oxide, polyglycolic acid(PGA), polylactic acid (PLA), and PGA-PLA copolymers.

What is claimed is:
 1. A method of manufacturing an expandable stentcomprising the steps of: braiding a plurality of resilient filamentsinto a tubular structure of a predetermined outer diameter; applying acurable material to the tubular structure; and collapsing the tubularstructure after applying the curable material to a low profile dimensionand curing the curable material to set the tubular structure in the lowprofile dimension.
 2. The method of claim 1 and further including thesteps of: placing the tubular structure in concentric relation over anuninflated balloon on a balloon catheter; and inflating the balloon toradially expand the tubular structure sufficient to destroy the curablematerial and allow expansion of the tubular structure.
 3. The method ofclaim 1 wherein the step of applying the curable material comprises:spraying the curable material onto the tubular structure.
 4. The methodof claim 1 wherein the curable material is selected from the groupconsisting of silicone, polyurethane, cellulose, flouopolymers,polyolefins, diene polymers and unsaturated polyesters.
 5. The method asin claim 3 wherein a water soluble powder is mixed with the curablematerial prior to its being sprayed onto the tubular structure.
 6. Themethod of claim 5 and further including the step of placing a waterimpermeable removable sleeve over the tubular structure.
 7. The methodof claim 5 wherein the water soluble powder comprises a medicament. 8.The method as in claim 1 wherein the curable material is applied bydipping the tubular structure into a polymeric solution.
 9. The methodas in claim 1 wherein the curable material is applied in a depositionprocess.
 10. The method of claim 1 wherein the tubular structure of thestent is self-expanding.
 11. The method of claim 1 wherein the tubularstructure of the stent is balloon expandable.
 12. The method of claim 1wherein the step of applying the curable material comprises coating thetubular structure with a curable material.
 13. The method of claim 1wherein the step of collapsing the tubular structure to a low profiledimension and curing the curable material to set the tubular structurein the low profile dimension comprises steps of: applying force to thetubular structure to collapse the tubular structure to the low profiledimension; and cross-linking the curable material to set the tubularstructure in the low profile dimension.
 14. A method of manufacturing anexpandable stent comprising the steps of: providing a tubular expandablestent of a predetermined outer diameter; applying a curable material tothe tubular stent; and collapsing the tubular stent after applying thecurable material to a low profile dimension and curing the curablematerial to set the tubular stent in the low profile dimension.
 15. Themethod of claim 14 wherein the tubular stent is self-expanding.
 16. Themethod of claim 14 wherein the tubular stent is balloon expandable. 17.The method of claim 14 wherein the step of applying the curable materialcomprises coating the tubular stent with a curable material.
 18. Themethod of claim 14 wherein the step of collapsing the tubular stent tothe low profile dimension and curing the curable material to set thetubular stent in the low profile dimension comprises the steps of:applying force to the tubular stent to collapse the tubular stent to thelow profile dimension; and cross-linking the curable material to set thetubular stent in the low profile dimension.
 19. A method ofmanufacturing an expandable stent comprising the steps of: providing atubular structure of a predetermined outer diameter; applying a curablematerial to the tubular structure; and collapsing the tubular structureafter applying the curable material to a low profile dimension andcuring the curable material to set the tubular structure in the lowprofile dimension.